Most of the code doesn't have to be loaded immediately. For example, you write in your comment, that Facebook can play 360 degrees videos, but the player doesn't have to be loaded until the user tries to play such a video.

I decided to look at Facebook code more closely. It is modular and contains several thousand of small modules. For example, on a news feed page the browser has loaded 66 JS files with total size of 5.7Mb containing 3062 modules [1].

But it is clear that many of those modules are not necessary to display a page. For example, a module with a name "XP2PPaymentRequestDeclineController" that is probably related to payments is not necessary. Or a module with a name "MobileSmsActivationController". Obviously Facebook preloads a lot of code for different dialogs that might be unused most of the times the page is loaded.

Of course I understand that it is very difficult to optimize code when a large team is contantly writing new code and everybody has strict deadlines so there is no time for optimizations, especially if they require serious refactoring.

[1] https://gist.github.com/codedokode/cb506cee367bdb9e1071bc186...